Sintering of metal elements



March 15, 1960 Filed June 21, 1957 F. C. WAGNER 2 Sheets-Sheet 1 LIQUID MIX POLYSTYRENE XYLENE 6O PARTS 2O GMS.

80:: BY WEIGHT STAINLE SS STEEL. MIX WAX METAL POWDER AT 5 PARTS 325 MESH 49C 60C BY WEIGHT I00 PARTS BY WEIGHT HEAT AND STIR IN PAN TO I9I C I96 C VENTING WATE R COOLING wAx AND RESIN BuRNEp OFF REUSE ALUMINA 60F DEW POINT ExTRA DRY HYDROGEN SI NTERING FURNACE SIN'ILERING FURNACE 400 C FOR I HOUR -TEMPERATURE MAINTAINED XYLENE DRIVEN OFF 0F T0 -|oF DEW POINT DRY HYDROGEN -COOL TO ROOM TEMPERATURE UNLOAD SHAKE FREE OF ALUMINA RETURN TO COOLING l230C INVENTOR. FIG. 1 FRANK c. WAGNER ATTORNEYS March 15, 1960 F. c. WAGNER 2,928,733

' SINTERING OF METAL ELEMENTS Filed June 21, 1957 j 2 Sheets-Sheet 2 FIG. 5

INVENTOR. FRANK C. WAGNER BY lf y m ATTORNEYS United States Patent This invention relates to improved porous sintered metal elements and methods of making such elements.

More particularly it relates to procedure for preparing elements by 'metal powder extrusion technique and for effectively sintering them.

' In the preparation of stainless steel elements stainless steel powder mixed with synthetic resin binder, organic solvent and a lubricant are extruded, for example, as tubing having convoluted section. The extruded tubing is then-cut i'nto green lengths and subjected to sintering procedure;

During sintering these elements require support becauselof the softening and burning off of the resin binder atlow temperature, which would result" in collapse of unsupported elements. To prevent this collapse, the elements are supported by embedding them in a setter of loosely packed refractory powder prior to elevation to sintering temperature; v

Experimental research has shown that the use of proper setter materials is absolutely essential because improper setter materials are found to react chemically with the metal of the green element or to cake or to adhere so strongly to the green element after sintering as to render it expensive and difiicult to produce clean setter free sintered elements. v

Objects and features of the present invention are the provision of satisfactory setter material for sintering operations and novel sintering procedure using such material to provide minimal adherence of setter material to the sintered elements andready removal of the small amountretained by simple mechanical brushing or the like.

Other objects and featurescof the invention will become apparent from the following description and accom panying drawings, wherein:

Figure 1 represents a flow diagram of a preferred method of practicing the invention;

Figure 2 illustrates in perspective a filter element pre pared in accord with the method;

While minus 325 mesh stainless'steel powder is preferred, powders of other dimensions are contemplated as useful. Likewise other steel, bronze, brass, titanium, zirconium, aluminum and other metal powders may be used.

follows:

Percent C 0.10 Si 2.37

Mn i 0.85 S v 0.008 Cr 18.11 Ni 8.19 Fe Balance Although as stated a wide range of particle sizes can be used, in accordance with the porosity required, a typical Ipowder useful in practicing the invention comprises particles in the 100 to 325 mesh range.

In preparing the powder for extrusion, it is found desirable to mix it with material that will assist in shaping it to final form and retain it in such form as a green element after extrusion. Accordingly, a heat depolymerizable organic binder is dissolved in a volatile aromatic hydrocarbon solvent and mixed with the powder. A

typically useful binder is polystyrene which is capableof being driven 011 during "heating without leaving carbo- Figure S'is a perspective, partially broken away view of a die used in extruding the element of Figure 2;

Figure 4 is a sectional elevation of extruding apparatus using the die of Figure 3; and V Figure. 5 is a perspective view partially in section of an extruded green element packed in setter material for sintering operations.

In a specific example of practicing the invention stainless steel powder of minus 325 mesh is used. A preferred powder-is 18Cr-8Ni type with the addition of about 2.5% silicon which latter permits sintering to occur. at temperatures below 1260 C. whereas non-silicon 'containing powders require at least 1315 C. i

naceous, tarry or gummy residues that would char during sintering. 1

The polystyrene is mixed with the solvent which may be xylene, at room temperature in the ratio of 20 grams of resin to 80 cc. of xylene. This forms a fairly viscous liquid to which is addedlOO parts by weight of minus 325 mesh stainless steel powder per parts by weight of the .liquid.

This mix is stirred thoroughly and warmed to 49-60 C. At this time 5 grams of a lubricant wax, for example, Cardis No. 319 die wax (a product of Warwick Wax Company, Long Island City, New York) is melted and added to the mix'which is then transferred to a pan. Further heating and stirring of the mix inthe pan is efiected at a temperature of from, 191-196 C. resulting in uniform distribution ofthe wax and driving an of more xylene until a plastic mass results having a putty like consistency. .The extruding apparatus is maintained in the range of extrusion temperature, namely, l9l-l96 C.

The mass is now ready for'extruding and is transferred to the extruding apparatus 11 of Figure 4. This apparatus 11 forms no part of the instant invention but"is-' merelyillustrative of the types of devices that can be used.

The heated mixture at 191-196 C. is placed in a heated die cylinder 12 which has a base 13 having a convoluted section outlet 14 in which a convoluted die 15 is mounted. A ram 16 at the upper end of cylinder 12 is operated by a power source (not shown) in order to' force the heated mixture mass through the limited convoluted opening 14 in base 13 past die 15. An electric heater coil 17 is positioned around cylinder 12 for maintaining the temperature of the mass mixture at from 191l96 C.

The die cylinder 12 and its die 15 are more fully shown in Figure 3. The die 15 as an insert is fastened to a spider 18 whose legs 19 are secured to the die base 13 so as to support the insert concentrically within the base opening 14. In the embodiment shown both the opening The composition of the preferredstai'nless steel is as 14 and the insert ,15 haveconvoluted sectionand are con,.

centrically spaced from each other.

The die cylinder 12 is installed as shown in Figure 4 so that the heated mass from cylinder 12 is extruded by ram 16 via the annular space defined between insert 15 and opening 14. The head pressure created by the ram is in the range of approximately 1500 p.s.i. This pressure is a variable factor, and may range from 100 p.s.i. upwardly depending on the desired porosity. It must be sufficiently high to fill the die space with -extr-udant A mass yet not be so great that the metal ,particles become too tightly packed during extrusion.

Cooling of the extrusion can be .efiected directly as it leaves the exit end of the die to prevent collapse by direct extrusion into water contained, for example, in

a water tank or jacket 20 which has a removable extrusion receiving core 21 onto which the hollow extrusion leaving the die base .13 and severed into, prescribed lengths is received.

When the extruded tube leaves the exit end of die base 13, it slides onto the core 21 in the water tank 20 moving thereover until cut into desired green units 22 of selected length, for example, by a hot wire or by been driven ofi and the metal powder particles are held together in a green or unconnected state by the resin and wax so that it is necessary to provide special sup-' 'port for them during sintering steps whose heat first melts. them then burns away the resin and wax.

In order to provide such support .each green unit 22 is placed in a container 24 in which it is completely packed in an inert refractory power preferably powdered alumina oxide. 'A satisfactory grade of such alumina is Tabular Alumina T-61 Grade, minus 60 mesh produced by Aluminum Company of America. This packing 25 is placed inside as well as outside of the green" element 22 in the container 24 to provide support for all convolutions but is packed loosely enough to prevent cracking due to shrinkage of the green units during sintering and also to permit free circulation of gas to the units as will be described. I

A container packed as-described is then placed in a batch type hydrogen furnace of conventional design whose details form no part of the invention. The furnace is slowly heated to approximately 400 C. during which polystyrene .binder and wax are destroyed and vented. Comparatively dry hydrogen having a dew point from 0 to minus 10 F. is circulated through the furnace at this time. A yellowish sooty flame occurs during this period. When all binder and wax are driven off the characteristic blue hydrogen fiame appears. This takes approximately 1 hour. The container is then cooled in the furnace to room temperature and the partially sintered element 22 removed therefrom and shaken free of the alumina refractory powder which does not stick to the unit 22 and placed in an unloaded or fresh container and returned to the hydrogen furnace for a second sintering stage.

The furnace nowis elevated to a sintering temperature of about 1230* C. and extra dry hydrogen having a dew point of approximately minus 60 F. is circulated through the furnace and in contact with the unsupported element 22 therein. This second sintering stage or operation is carried. on from 1 /2 to 4 hours and effectively sinters the steel powder particles together into a strong element.

Thereafter the element is cooled to room temperature in the furnace and removed. If by chance some residual alumina refractory is found clinging to the sintered element, it may be removed by tumbling in a tumbling barrel containing hot dilute sodium hydroxide (NaOH) solution long enough to dislodge such particles and otherwise reciprocating cutter 23 and each green unit 22 is now 7 ready for the next step in the process. It is to be ,dis-

clean the element.

The elements are then washed, air dried and ready for use.

The dryness of the hydrogen used during the two sintering steps is critical in obtaining good strength in the sintered elements particularly since high pressure is not used during extrusion. Since the extrusion pressure neither deforms the powder particles nor forces intimate contact between them, all particles being enveloped in a thin film of resin, oxide films on the particles are not ruptured during'extrusion. Hence the moisture content of the hydrogen is critical to prevent sticking. When the first sintering step "occurs the hydrogen -(0 to 10 F. dew point) is moister than that usedin the second sintering step (60 F. dew point hydrogen). This moister hydrogen prevents the alumina from sticking to the preliminarily sintered element. I

Inasmuch as the alumina is shaken away from the preliminarily sintered element before the second stage of sintering with dry hydrogen (60 F. dew point) is effected, the problem of clinging alumina is substantially eliminated and great strength can be imparted to the element by second stage sintering in the extradry hydrogen for periods up to 4hours.

Should some residualalumina powder be found clinging to the second stage sintered elements, the dilute acid leaching described can be employed if stiff brushing will not remove it. In the alternative, steam blasting or sodium hydride oxide reduction can be used to rid the s sinteredelements of such residual refractory.

Porosity of the. product is controllable by regulation of particle size, proportion of powder and binder temperature and time of sintering.

While a preferred embodiment of the invention has been described and shown, variations within the scope of the appended claims are possible and are contemplated. There is no intention, therefore, of limitation to the exact disclosure herein made.

What is claimed is:

1. In a process of preparing a sintered metal element from a green unit of extruded metal powder-organic binder mix, the steps comprising supporting the green unit in refractory alumina powder and effecting first stage sintering of the powder supported unit at a temperature of approximately 400 C. in an atmosphere of dry hydrogen Whose dew point ranges from 0 to 10 F. during which the organic binder is burned off, then removing the unit from the alumina powder support, ridding it of clinging alumina powder and thereafter effecting second stage sintering of the unsupported unit at a temperature of about 1230 C. in an atmosphere of extra dry hydrogen whose dew point approximates -60 F. for a period of from 1 /2 to 4 hours.

2. In a process of preparing a stainless steel sintered element from a green unit of extruded stainless steel powder-organic binder mix, the steps comprising supporting the green unit in refractory alumina powder and effecting first stage sintering of the powder supported unit at a temperature of approximately 400 C. for about one hour in an atmosphere of dry hydrogen whose dew point ranges from 0 to 10 F. during which the. organic binder is burned ofi, then removing the unit from the alumina powder support, ridding it of clinging alumina powder and thereafter effecting second stage sintering of the unsupported unit at a temperature of approximately 1230 C. for a period of from 1 /2 to 4 hours in an atmosphere extra dry hydrogenwhose dew point approximates -60 F.

Mr 131* v 3. A process'of preparing sintered metal elements comprising the steps of extruding at a pressure of between 100 p.s.i. and approximately 1500 psi a green element of powdered metal and organic binder into desired shape, packing the green element in a refractory powder support, eifecting first stage sintering of the powder packed element in an atmosphere of dry hydrogen having a dew point of from 0 to 10-F., removing the first stage sintered element from its powder packing, rid- .ding it of clinging powder packing, and efiecting second stage sintering of the first stage sintered element in an 1,008,806 Farkas Nov. 14, 1911 7 1,922,038 Hardy Aug. 15, 1933 2,152,006 Welch Mar. 28, 1939 2,471,630 Kurtz May 31, 1949 2,719,786

Fredenburgh Oct. 4, 1955 

1. IN A PROCESS OF PREPARING A SINTERED METAL ELEMENT FROM A "GREEN" UNIT OF EXTRUDED METAL POWER-ORGANIC BINDER MIX, THE STEPS COMPRISING SUPPORTING THE "GREEN" UNIT IN REFRACTORY ALUMINA POWDER AND EFFECTING FIRST STAGE SINTERING OF THE POWDER SUPPORTED UNIT AT A TEMPERATURE OF APPROXIMATELY 400*C. IN AN ATMOSPHERE OF DRY HYDROGEN WHOSE DEW POINT RANGES FROM 0* TO -10*F. DURING WHICH THE ORGAIC BINDER IS BURNED OFF, THEN REMOVING THE UNIT FROM THE ALUMINA POWDER SUPPORT, RIDDING IT OF CLINGING ALUMINA POWDER AND THEREAFTER EFFECTING SECOND STAGE SINTERING OF THE UNSUPPORTED UNIT AT A TEMPERATURE OF ABOUT 1230*C. IN AN ATMOSPHERE OF 